1. Field of the Invention
The present invention relates to a watermark information embedment device capable of adding secret information to a document image, the secret information being prepared in the form of other than characters, and also relates to a watermark information detection device capable of detecting the secret information from a print document in which the secret information is embedded
2. Description of the Related Art
In order to prevent the original data of an image and a document from being unfairly copied or forged, a digital watermark is used for embedding the information and the secret information in the original data of the image and the document in such a form that is made invisible to anyone except a specially authorized person. In case of the digital watermark, preservation and giving/receiving of the data are all carried out through the electronic medium, so that there hardly occur deterioration and lost of the information as embedded in the form of the digital watermark, thus enabling the information detection to be surely and accurately executed with ease. Similar to the case of the digital watermark for protecting the digital image and document, it is needed for an original image and a document printed on a paper medium, so-called a printed matter, to be prevented from unfair actions such as copying with ill intent, alteration, misappropriation, forgery, and so forth, by a certain means capable of embedding the secret information in the form of other than characters that are made invisible to anyone other than a specially authorized person.
With regard to a prior art method for embedding a watermark in a white-black binary document which is most widely utilized as a printed matter, there have been made the considerable number of prior art disclosures so far. Therefore, some of them will be described for reference in the following.
(1) Japanese Patent Publication No. 2001-78006 entitled “Method for Embedding and Detecting Watermark Information in White-Black Binary Document Image and Device Using The Same.”
A minimum rectangle surrounding an arbitrary character string is divided into a certain number of blocks, which are further divided into two groups, group 1 and group 2 for instance (the number of groups may be 3 or more). When a signal is “1”, the characteristic quantity in each block of the group 1 is increased and the characteristic quantity in each block of the group 2 is decreased. When a signal is “0”, reverse operation is carried out. In this case, the characteristic quantity in the block indicates the number of pixels and the thickness of characters in the character region, a distance needed for a scanner to run before meeting with the first character region while it vertically scans the block, and so forth.
(2) Japanese Patent Publication No. 2001-53954 entitled “Information Embedment Device, Information Readout Device, Digital Watermark System, Information Embedment Method, Information Readout Method and Storage Medium.”
In this disclosure, the width and height of a minimum rectangle surrounding one character is defined as a characteristic quantity and a symbol is indicated by a classification pattern classified based on the size (large/small relation) of the characteristic quantity between two or more characters. For instance, six characteristic quantities can be defined from three characters and possible combination patterns are enumerated based on the size (large/small relation) of the characteristic quantities. These combination patterns are further divided into two groups, of which each is given a symbol. If the information to be embedded is “0” and the character quantity of a character selected to indicate this information is “1,” any one of six characteristic quantities is changed by expanding the character region or so. The pattern to be changed is selected such that the quantity of the change becomes minimum.
(3) Japanese Patent Publication No. 9-179494 entitled “Method for Recording Secret Information.”
In this disclosure, it is presumed that images and documents are printed by means of a printer of the 400 dpi type. The information is converted into a numerical form and is expressed by a distance (the number of dots) between a reference mark and a position distinguishing mark.
(4) Japanese Patent Publication No. 10-200743 entitled “Document Processing Device.”
In this disclosure, the information is expressed by judging whether or not screen lines of a million line screen (a special screen made up of a lot of thin parallel lines) are moved backward.
According to the above prior art disclosures (1) and (2), however, the pixels constituting the characters of the document image, interval between characters and interval of lines will have to be changed, so that it becomes necessary to change fonts and layouts. In addition, according to the above prior art disclosures (3) and (4), when detecting the information, it is necessary to carry out such a high accuracy detection that the information embedded in the input image as read out by such a device as a scanner is detected with very high accuracy of one pixel unit. In spite of the demand for such high accuracy detection, if very fine contaminants are mixed in a printing paper, or if certain noises are introduced while the document image is printed or read out, the accuracy of the information detection would come to receive a large influence.
As described above, according to the above prior art disclosure (1) through (4), even if trying to detect the secret information embedded in a printed document image by first inputting the document image to the input device like a scanner and further inputting the result of scanning to a computer, so many noise components would be involved in the data of the document image inputted to the computer, because of stains left on the printed document image, deformation of the image due to rotation of the image possibly occurring when first setting the printed document image to the input device. Therefore, it becomes so hard to fetch the secret information with accuracy, even though making use of the above prior arts techniques as described above.